A composite bridge amplifier is disclosed in U.S. Pat. No. 5,075,634. This amplifier comprises a combination of a single-ended Class D amplifier and a linear Class AB bridge amplifier. The output of the Class D amplifier provides the supply voltage for the bridge amplifier. This supply voltage is dependent on the signal to be amplified, producing a small voltage drop across the output voltage sources in the bridge amplifier. This substantially reduces the power dissipated in the voltage sources, providing a highly efficient amplifier with Class AB type performance.
A second composite bridge amplifier is disclosed in U.S. Pat. No. 5,510,753. This amplifier provides a closed loop control system for the Class D amplifier, improving the performance of the amplifier and eliminating the need for a time delay element which was required in the prior art. In addition, this amplifier incorporates current overload protection and thermal overload protection stages which protect the output stage.
In each of these known composite bridge amplifiers there are signal conditioning electronics and control electronics which are powered by separately generated low voltage power rails. Generation of these independent rails requires the use of a power transformer and additional circuitry. The cost of the composite bridge amplifier may be reduced by generating the low voltage power rails from the power rails used to power the bridge amplifier.